Mechanistic studies are being performed on the enzyme purine Nucleoside Phosphorylase from various sources to decide whether phosphorolysis takes place by an A1 (oxocarbonium ion) or A2 (direct displacement) pathway. Tools are developed to look at the transition state of phosphorolysis. Affinity labels (active site directed irreversible inhibitors) are synthesized to covalently label the catalytic site of the enzyme with a view to eventual elucidation of the amino acids involved in binding and catalysis. The labels to be synthesized take advantage of the inhibitory properties (earlier demonstrated) of 9- and 7-alkylguamines and hypoxanthines earlier. Functional groups to be introduced will be directed towards those amino sidechains whose chemical modification had earlier been shown to inactivate the enzyme.